SYNKINESIAS: CHARACTERISTICS, TYPES, EXAMPLES - MEDICINE - 2025

The term synkinecia is used to describe the involuntary muscle contractions that are associated or occur concomitantly with the execution of a voluntary movement. Syncinecies occur in muscles or muscle groups other than those that are participating in voluntary movement.

Synkinesias are therefore involuntary movements associated with other movements that are directed by the will and whose development is oriented with an intention towards a specific end.

Involuntary face spasms (Source: Unknown author / Public domain, via Wikimedia Commons)

Although this type of associated movement usually occurs automatically, it cannot be considered a reflex. It is not triggered by a sensory or afferent stimulus or excitation, but is the parallel result of the same neuronal or efferent motor excitation that gives rise to the voluntary movement with which it is associated.

Synkinesis characteristics

There are two main characteristics of synkinesias and they are already mentioned in their definition. On the one hand, they are automatisms, that is, they are motor actions that occur without the help of the will. On the other, they are always associated with some other voluntary movement in particular.

The term "synkinesias" is used mainly in clinical language to indicate conditions such as the one described and that occur in neurological disorders. They are, in this sense, unwanted, but rather undesirable, motor manifestations or symptoms, whose appearance is considered disturbing.

Types of synkinesias

The term synkinesis allows to include in the concept any motor activity that is associated with another that represents the main objective of intentionality. In this sense, one could speak of physiological and pathological synkinesis.

- Physiological synkinesias

These would be the motor activities that are normally associated with specific activity patterns aimed at producing a particular movement of some body segment or segments, but which are not directly related to said movement.

Postural adjustments

Examples of this type of physiological synkinesis would be the different involuntary postural adjustments that are associated with and are necessary for the normal development of any specific motor activity that involves the contraction of skilled muscles.

When we sit down at the table and put food in our mouths, or when manipulating the keyboard of a computer or a telephone, and in general, in any skillful motor activity that is carried out, there is a main motor activity of whose beginnings we are aware and our will is responsible.

There is also a series of secondary motor adjustments in other muscle groups, which we do not order, but which are included in the motor program that we voluntarily start up and which are essential for the development of the main motor activity.

All this secondary motor activity included in the neural motor pattern of a voluntary movement is destined to execute muscular adjustments that allow the conservation of the corporal posture and the balance, as well as the stabilization of the member (s) that executes (n) the main movement.

Physiological synkinesias of childhood

During childhood and almost until the onset of puberty, there are a series of synkinesias that can be considered normal or physiological, and that appear in the child as a result of the relative immaturity of the nervous system during the growth and development stage. They usually disappear completely after 11 years of age.

Examples of this type of synkinesis: protrusion of the tongue out of the mouth when writing. The production of symmetrical, mirrored movements in the contralateral hand, when the other hand performs movements such as the alternating pronation and supination that occur when moving a puppet.

The latter synkinesis can even persist into adult life and be considered normal in certain individuals, as long as it is not accompanied by other pathological manifestations.

Another normal synkinesis present in the adult and considered physiological is the displacement in the opposite direction of an upper limb when, when walking, the ipsilateral lower limb moves forward or backward.

- Pathological synkinesias

They are those that appear as a result of some neurological disturbance, especially after injuries that cause damage to the distal regions of the nerve fibers of a motor nerve. Especially if said damage, which can be degenerative or traumatic, involves the interruption or section of the nerve fibers.

After an injury of this type, regeneration processes of interrupted nerve fibers occur. Said regeneration involves the growth of the proximal portion of the fibers that still remain attached to their neuronal bodies; growth and prolongation that attempt to reestablish connections with the muscle fibers that originally innervated.

The condition for synkinesis is established when some of the collaterals that sprout from the regenerating axons, instead of following the path towards 'their' original muscle fibers, follow an aberrant course that takes them towards another muscle group.

Any nervous excitatory activity that descends through the affected pathway and that is directed to produce the movement that it originally triggered, will also activate these collaterals and the “foreign” muscle fibers that mistakenly received them, which would account for the production of a synkinesis that would involve to the wrongly "reinnerved" muscle group.

Ephaptic transmission

Other explanations for the production mechanisms of synkinesis have been postulated. One of them involves the establishment of non-synaptic contacts between neighboring nerve fibers, in such a way that the nervous excitation that circulates through one of them ends up passing to the neighboring ones, due to the close contact between both membranes, and going to other muscles. This is called ephaptic transmission.

Hyperexcitability of neurons

Another likely explanation is the hyperexcitability of certain neurons in the motor nuclei, which, when denervated, develop denervation hypersensitivity. This means that collaterals coming from axons directed to other motor neurons and without effectiveness before the injury, can now cause the excitation of those neurons that would innervate other muscles.

Examples

Pathological synkinesias

Examples of pathological synkinesias would include those that occur in the limbs, which are rare, and those that occur at the level of the facial muscles and the external muscles of the eyes. Muscle groups most often affected.

Limb synkinesia

Among the limb synkinesias we can mention bimanual synkinesias, which occur when when executing a movement of the hand and fingers on one side, the hand and fingers on the other side reproduce the same movement.

These synkinesias can be pathological in association with Parkinson's or Kallman's syndrome, which is a kind of hypogonadotropic hypogonadism.

Synkinesias of the face

Synkinesias of the face include those that involve sequelae of facial nerve damage, such as the one that occurs in facial palsy or Bell's palsy, which causes diffuse degeneration and demyelination of the facial nerve or VII cranial nerve, and that probably It is due to a viral reactivation.

Left side of the face affected by Bell's palsy (Source: Shantoo / CC0, via Wikimedia Commons)

Synkinesias of the facial nerve

The facial nerve controls most of the skeletal muscles of the face, but it also innervates other structures such as salivary, sweat, and tear glands. The synkinesias produced can involve combinations between different voluntary movements and different synkinesias.

Examples include:

• The involuntary closing of the eyes when muscles of the mouth are voluntarily contracted.
• Movements of the middle part of the face with the voluntary closing of the eyes.
• Crocodile tears, which are the intense unilateral tearing that occurs when eating.

Synkinesias of the extraocular muscles

These muscles are six:

• The superior, inferior and internal rectus and the inferior oblique are innervated by the common ocular motor (third cranial nerve).
• The superior or greater oblique innervated by the pathetic (IV pair).
• The external rectum innervated by the external ocular motor (VI pair).

After trauma to these nerves, synkinesias can occur that involve combinations of movements of the eyeball that include those produced by any pair of them.

Furthermore, as the 3rd nerve has many functions, including eyelid retraction and pupillary contraction, these functions can also be included in synkinesis.

Examples

• When abduction or deviation of the eyeball away from the midline is attempted, the eye rather approaches the midline and the eyelid is retracted.
• When adduction or deviation of the eye towards the midline with descent of the same is attempted, the eyelid is retracted.

References

1. Faris C, Sauvaget E: Management of Facial Palsy, in: Nerves and Nerve Injuries, 2015.
2. McGee S: Complications of Bell Palsy, in: Evidence-Based Physical Diagnosis (3rd Edition), 2012.
3. Rubin DI: Clinical Neurophysiology: Basis and Technical aspects: Synkinesis, in: Handbook of clinical Neurology, 2019.
4. Ryan MM, Engle EC: Disorders of the Ocular Motor Cranial Nerves and Extraocular Muscles, in: Neuromuscular disorders of Infancy, Childhood, and Adolescence (2nd Edition), 2015.
5. Victor, M., Ropper, AH, & Adams, RD (2001). Principles of neurology (Vol. 650). New York: McGraw-Hill.